(1) Field of the Invention
The present invention relates to a liquid crystal display device, particularly to an in-plane switching (hereinafter referred to as “IPS”), semi-transmissive liquid crystal display device.
(2) Description of the Related Art
Semi-transmissive liquid crystal display devices that have a transmission section and a reflection section in a subpixel have been used as displays for portable devices.
These semi-transmissive liquid crystal display devices employ the vertical electric field method in which liquid crystal interposed between a pair of substrates is driven by applying an electric field in a direction perpendicular to the planes of the pair of substrates. Those display devices have a step between the transmission and reflection sections so as to match the properties of the transmission section and those of the reflection section with each other, as well as have a phase difference plate between a polarizing plate and a liquid crystal layer.                As liquid crystal display devices, IPS liquid crystal display devices have been known. These IPS liquid crystal display devices have pixel electrodes PIX and counter electrodes CT formed on the same substrate, and apply an electric field between each pixel electrode PIX and counter electrode CT to rotate the liquid crystal in the substrate plane in order to control the contrast. Thus, IPS liquid crystal display devices have a characteristic that tints of displayed images are not inverted when seen obliquely. To utilize this characteristic, it has been proposed to construct a semi-transmissive liquid crystal display device using an IPS liquid crystal display device, for example, in Japanese Patent Application Laid-Open Publication No. 2003-344837.        
However, as also described in Japanese Patent Application Laid-Open Publication No. 2003-344837, when a semi-transmissive liquid crystal display device is constructed using an IPS liquid crystal display device, for example, there has been a problem that the transmission section is placed in normally white mode and the reflection section is placed in normally black mode, whereby the contrast are inverted between the transmission and reflection sections.
In order to address the abovementioned problem, the applicants have already filed a patent application for a semi-transmissive liquid crystal display device having a new pixel structure (see Japanese Application No. 2005-322049).
Each pixel of this semi-transmissive liquid crystal display device applied for patent is structured so that the transmission and reflection sections share a pixel electrode while each having an independent counter electrode. Thus, applying different reference voltages (counter voltage and common voltage) to the independent counter electrodes prevents inversion of the contrast between the transmission and reflection sections.
In this semi-transmissive liquid crystal display device applied for patent, the transmission section has a normally black characteristic (the section is displayed in black when no voltage is applied), and the reflection section has a normally white characteristic (the section is displayed in white when no voltage is applied).
On another hand, in an IPS liquid crystal display device, a pixel electrode PIX is disposed on the side adjacent to liquid crystal, of one of a pair of substrates. In this case, a hole is made in a counter electrode CT to form a contact hole so that a drive voltage is applied to the pixel electrode PIX via the contact hole (see Japanese Patent Application Laid-Open Publication No. 2002-328385).
However, in the liquid crystal display device described in Japanese Patent Application No. 2005-322049, when a hole is made to form a contact hole in order to apply a drive voltage to the pixel electrode PIX disposed on the side adjacent to the liquid crystal of the one substrate, as disclosed in Japanese Patent Application Laid-Open Publication No. 2002-328385, non-display sections are increased, thereby reducing the transmissivity.
In other words, when a contact hole for applying a drive voltage to the pixel electrode PIX is formed according to a related-art layout in the semi-transmissive liquid crystal display device applied for patent, non-display sections are increased, resulting in a reduction in transmissivity.